On the heels of S&P Global announcing Marathon Digital would be added to a small-cap stock index, the bitcoin miner's shares rose by 18% on Monday, boosting the company's market capitalization by about $800 million, according to Ycharts data.

Marath0n Digital's shares closed the day's trading at $20.67, according to Yahoo Finance. The company's market capitalization rose to $5.5 billion from Friday's market capitalization of $4.7 billion, according to Ycharts data.

After Friday's trading session closed, S&P Global said Marathon Digital would be added to its S&P SmallCap 600.

"The index is designed to track companies that meet specific inclusion criteria to ensure that they are liquid and financially viable," according to S&P Global.

Also on Monday, the company filed an …